bpo-41744: Package python.props with correct name in NuGet package (GH-22154)

bpo-41744: Package python.props with correct name in NuGet package (GH-22154)

NuGet automatically includes .props file from the build directory in the
target using the package, but only if the .props file has the correct
name: it must be $(id).props

Rename python.props correspondingly in all the nuspec variants. Also
keep python.props as it were for backward compatibility.